sweet/benchmarks/internal/driver: don't swallow perf stderr

It took me way too long to figure out why I wasn't getting perf data
when I had forgotten to enable the magic sysctls and wasn't getting
the usual hint from perf.

Change-Id: I5e518b024dccff45ff06daa84ac866708c1d4ddc
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/600061
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
1 file changed